Modems like Digital Subscriber Line (DSL) modems are normally connected to a device for providing access to a data communication network. Such a device is typically a Digital Subscriber Line Access Multiplexer (DSLAM) and the communication network may be the Internet. One particular type of standard that may be used is Very-high-speed Digital Subscriber Line 2 (VDSL2).
When being connected in this way the modems are connected to the device via separate transmission lines, typically conductor pairs made of copper. These transmission lines are furthermore often standard telephone lines, often denoted Plain Old Telephone Service) POTS lines and typically made of twisted copper pairs.
The properties of these transmission lines are normally not known beforehand, especially with regard to data communication, but may be obtained through applying line measurements of these transmission lines.
One way of obtaining measurements is through Single-ended line testing (SELT), which is a cost-effective technique which DSL operators utilize since it requires only measurements from one end of the copper line. This is made possible in practice by using pre-existing equipment as e.g. POTS line cards and/or DSL modems. Typical measurements may include the total line capacitance at DC (zero frequency), the line input impedance, and the reflection coefficient (i.e. scattering parameter S11).
The measurements may then be used in determining transmission line parameters. Several kinds of line parameters may here be of interest, for instance, line resistance, inductance and capacitance.
One property of the transmission line that is of interest is the capacity to transfer data. The capacity may be obtained through determining the maximum number of bits the line can support for a given error rate. The capacity can be estimated from the frequency dependent transfer function of the line (i.e. line attenuation) and the total noise (i.e. thermal plus crosstalk noise). The total line noise typically changes over time but can often be directly measured or assumed to comply with some crosstalk scenario. The transfer function, however, can be considered time invariant and needs to be estimated only once or seldom.
This transfer function is however normally not known and may therefore have to be obtained through some of the above mentioned transmission line parameters.
There is therefore a need for determining transmission line parameters.
One approach for determining of transmission line parameters in the field of microwave transmission is described by Martin Norgren and Sailing He in “An optimization approach to the frequency-domain inverse problem for a nonuniform LCRG transmission line”, IEEE Transactions on microwave theory and techniques, vol. 44, No. 8, August 1996.
Here parameters are determined based on knowledge of the length of the transmission line where the meaning of transmission line in the microwave application refers to e.g. a microstrip or a filter with a physical extension fraction of a meter. This length is in many cases, and typically in the case of POTS/DSL lines, unknown and hard to determine accurately. For the POTS/DSL line case, this length is in the order of tens of meters up to several hundreds of meters, and therefore differs significantly from the transmission medium used for microwave applications.
There is therefore a need for determining transmission line parameters without having to consider the length of the transmission line.
The present invention is directed towards solving this problem.